This invention relates generally to assembly machines which drill/countersink openings in aircraft skins and other large assemblies and which shave the tops of rivets which have been previously installed in such openings, and more specifically concerns such a machine which has the capability of compensating for changes in the apex (farthest point) of the stroke of the spindle which holds the drill/shave tools caused by mechanical and thermal effects encountered during operation of the machine.
Large, complex assembly machines, such as shown in U.S. Pat. Nos. 5,033,174 and 5,699,599, are used to manufacture large mechanical assemblies, such as the wing portion of large commercial aircraft. The assembly machines perform operations which include drilling holes in the mechanical assemblies for insertion of rivets and bolts to serve two or more parts of an assembly together, such as the skin and struts of an aircraft wing. These operations, involving the use of tools which rotate at high speeds, include drill-countersink operations, drill-chamfer operations and drill-rivet shave operations. A high degree of accuracy in the above three operations is quite important in many assembly operations, particularly those involving large aircraft assemblies, for which the present invention was designed.
The outside surface of an aircraft, referred to as the aircraft skin, is desirably flat and smooth. This reduces drag and hence fuel consumption when the aircraft is in operation. Further, such characteristics of the aircraft skin are important to aircraft appearance and resulting customer satisfaction. For these reasons, very accurate machine operations on the skin of aircraft assemblies such as wings are quite important. For instance, for bolt installations which require a countersink, the tolerance for the countersink, including the manufacturing tolerance in the bolt, must be such that the resulting dimensional variance between the skin and the surface of the bolt is within the range of 0.000 to xe2x88x920.007, for a tolerance of xc2x10.0035. Since the bolt manufacturing tolerance is xc2x10.002 inches, the resulting tolerance for the depth of the countersink is xc2x10.0015.
Another machine operation involves riveting. In riveting operations, skin surface smoothness is controlled by rivet shaving. Rivet head shaving operations in aircraft assembly must typically be within a dimensional range of 0 to +0.002, for a tolerance of xc2x10.001, between the skin surface and the head surface of the shaved rivet.
While the primary focus of this invention is directed toward highly accurate machine operations, including drill countersink and rivet shaving on aircraft skin surfaces, it should be understood that the invention is also applicable for determining tool position relative to the workpiece for other tools used in the manufacture of other large assemblies where high accuracy is important. For instance, a precision chamfer is necessary in many aircraft (and other) assembly situations for a proper, tight interference fit between the inside corner of bolts, i.e. the area where the head of the bolt meets the shank, and the opening in the workpiece.
Accordingly, accurate control over drilling operations, including knowledge and control over the farthest point of movement of the tool, referred to herein as the apex point, and the ability to compensate for changes due to various operating factors, is quite important in achieving the objectives of accurate machine operations.
The location of the tip of the machine tool at the end of the spindle stroke determines the depth of the machine operation, i.e. countersink, chamfer or shaving. Again, this depth must be controlled quite accurately because of the extremely tight tolerances described above for such operations.
In basic operation of such an assembly machine, a cutting tool such as a drill with a countersink or chamfer arrangement, or a shaver, is mounted in a spindle assembly. The spindle assembly typically includes a servo-controlled ball screw or a servo-controlled linear motor to move the tool holder portion of the spindle toward the workpiece in a controlled manner. In some cases, a linear scale mechanism is included as part of the spindle assembly to increase accuracy. Existing systems, however, are subject to both mechanical and thermal changes and/or errors, which decrease accuracy. Mechanically, such changes/errors include contact errors between the face of the pressure foot of the machine tool and the skin surface of the assembly. In addition, chip residue from the drilling operation can accumulate between the pressure foot and the skin surface. Curvature or other variation in the shape of the work surface, including deflection and/or rotation of the pressure foot, can also produce mechanical errors.
Another source of error is change in temperature of the spindle assembly, which results in what is referred to as thermal growth of the various portions of the spindle assembly. The dimensions of the spindle assembly will actually change sufficiently because of increase in temperature to affect the accuracy of machine operations. Thermal growth occurs throughout the spindle assembly and is not necessarily uniform or continuous along the length of the spindle assembly, due to various factors, as described in more detail below.
In existing systems, the control computer for the machine has the capability of evaluating thermal data from temperature sensors and the like positioned on the spindle assembly, and can adjust the spindle stroke accordingly. However, it is quite difficult to position temperature sensor(s) on the rotating spindle shaft portion of the assembly. Temperature gradients and discontinuities and the resulting growth in the various portions of the spindle assembly are difficult to predict and hence measure and thus are very difficult if not impossible to compensate for.
While certain techniques have been developed to counter/compensate for the effects of thermal growth and mechanical errors, they have proven to be not very effective, particularly in meeting close tolerance requirements. For instance, attempts to control temperature include cooling the spindle during operation, or running the spindle (warm-up) for a considerable time prior to use. Other possibilities include using a linear scale, as indicated above, relative to the feeding of the spindle to eliminate errors back of the readhead, as well as attempting to measure temperature fluctuations at various points along the spindle assembly and compensating for the resulting tool length variations.
The biggest issue with thermal growth concerns the tool holder and the actual tool or cutter. When the tool holder and the cutting tool stop turning, they increase in temperature due to heat conducted forwardly from the spindle body. Once the machine is in operation, however, the tool holder and cutter are exposed to a xe2x80x9cwindxe2x80x9d effect created by the high speed turning of those elements. This effect is the source of a thermal discontinuity which results in the compensation for tool length changes using existing thermocouples not matching the actual thermal growth of the tool.
It has proven difficult to effectively cool those portions of the spindle assembly when the system is not in operation, and keeping the tool running for a substantial period of time prior to actual use is undesirable due to energy and safety considerations. It is preferred that the tool be running only when it is actually used for cutting. Further, it is very difficult, if not impossible, to measure the temperature of the forward portions of the spindle assembly, i.e. the tool holder and the tool itself, which, depending upon the particular operation, can be turning at between 6,000 and 20,000 rpm.
Hence, it is desirable to be able to determine and compensate for the apex position of the tip of the cutting tool during the spindle stroke. For best results, both mechanical and thermal sources of change must be determined and compensated for.
Accordingly, the present invention is an apparatus and corresponding method for performing accurate machine operations on mechanical assemblies, comprising: a first spindle for holding a cutting tool for performing very accurate machine cutting operations, such as countersunk holes; a moving assembly for moving the first spindle toward and away from a workpiece, the workpiece comprising at least two workpiece elements which are to be joined together, the workpiece having a near surface relative to the cutting tool, wherein the cutting tool has an apex position in its movement relative to the workpiece; a pressure foot assembly for clamping the workpiece elements and stabilizing them in a fixed position, wherein the pressure foot assembly has an aperture through which the cutting tool can move to contact the workpiece; an element which is mountable on the first spindle for determining the position of the near surface of the workpiece relative to the cutting tool along a line which is coincident with the central axis of the cutting tool; and means for adjusting the apex position of the cutting tool in accordance with the determined position of the workpiece relative to the cutting tool to achieve a very accurate cutting operation in the workpiece.